Generally, lens actuating devices for video cameras and digital still cameras having an automatic focusing function and an electronic zooming function have a lens position detecting device for detecting the position of a movable lens for automatic focusing and the position of a movable lens for zooming.
Relatively many lens position detecting devices of the above type employ a magnetoresistive device such as an MR sensor for converting a change in the magnetic force of a magnet, for example, into an electric signal.
For example, there has been proposed according to prior art 1 a lens position detecting device having a position detecting magnet that is magnetized into alternate different magnetic poles along the direction in which a movable member moves, and a magnetoresistive device fixed to a fixed member in confronting relation to the range in which the position detecting magnet is movable, the magnetoresistive device having its resistance variable depending on a change in magnetism (see, for example, Japanese Patent Laid-Open No. 2002-169073).
The lens position detecting device requires a magnet having a size that is substantially equal to the moving stroke of the movable member. The positional signal produced by the magnetoresistive device is represented by a repetitive wave having constant amplitude.
According to prior art 2, there has been proposed a lens position detecting device which employs a Hall device instead of the MR sensor (see, for example, Japanese Patent Laid-Open No. Hei 11-149030).
The lens position detecting device has a magnetic scale magnetized into alternate S and N poles arranged at a predetermined pitch along a driving direction, and a magnetic sensor installed in confronting relation to the magnetic scale with a certain distance provided therebetween. The magnetic sensor includes an MR device, a Hall device, or the like,
According to prior art 3, there has also been proposed a position detecting method which employs an inclined magnet and a Hall device to detect a position based on a change in the magnetic flux applied to the Hall device when the distance between the inclined surface of the magnet as a movable member moves in the direction of an optical axis, i.e., in an optical axis direction (see, for example, Japanese Patent Publication No. 2881959).
Image capturing devices such as digital still cameras, digital video cameras, etc. have a lens barrel.
The lens barrel has a lens for guiding a subject image to an image capturing unit, the lens being disposed forwardly of the image capturing unit which captures the subject image, a lens holder frame for holding the lens, and an actuating mechanism for moving the lens holder frame in an optical axis direction. The lens, the lens holder frame, and the actuating mechanism are disposed in a barrel (see, for example, Japanese Patent Laid-Open No. 2002-296480).
The actuating mechanism has a male screw member disposed radially outwardly of the lens and extending in the optical axis direction, a motor for rotating the male screw member, a support member supporting the both ends of the male screw member, and a female screw member threaded over the male screw member and movable in the direction in which the male screw member extends. The female screw member is connected to the movable lens. When the motor is energized, the movable lens is moved in the axial direction in unison with the female screw member.
The motor includes a rotary motor having a rotor rotatable with respect to a stator.
There is also proposed a lens barrel which employs a linear motor, instead of the rotary motor, having a stator and a rotor which are developed in a linear configuration.
FIG. 72 is a perspective view of an arrangement of a lens barrel employing a linear motor and FIG. 73 is a cross-sectional view of FIG. 72.
As shown in FIGS. 72 and 73, the lens barrel has a base 2 with an image capturing unit 1 mounted therein, a lens holder frame 4 holding a lens 3 for guiding a subject image to the image capturing unit 2, two guide shafts 5 inserted respectively through two bearings 4A of the lens holder frame 4 and holding the lens holder frame 4 movably in the optical axis direction of the lens 3, a linear motor 6 as an actuating mechanism for moving the lens holder frame 4 in the optical axis direction of the lens 3, and a position detecting mechanism 7 for detecting the position of the lens in the optical axis direction thereof.
The linear motor 6 includes a voice-coil linear motor, and has a coil 6A fixed to the lens holder frame 4 and having a winding around a winding axis parallel to the optical axis, a yoke 6B inserted centrally through the coil 6A and fixed to the base 2, and a magnet 6C magnetized into N poles and S poles along a direction perpendicular to the winding axis.
The yoke 6B includes a back yoke 6B1 extending parallel to the optical axis and supporting the magnet 6C mounted thereon, and an opposite yoke 6B2 extending parallel to the back yoke 6B1 in spaced relation thereto and inserted centrally through the coil 6A. Since the back yoke 6B1 and the opposite yoke 6B2 have their ends interconnected, the yoke 6B and the magnet 6C make up a closed magnetic circuit. Any magnetic fields generated outside of the magnetic circuit can almost be ignored.
When a drive current flows through the coil 6A, it generates a magnetic field, and the magnet 6C and the yoke 6B generate a magnetic field. These magnetic fields magnetically interact to apply a drive force to the coil 6A, i.e., the lens holder frame 4, in the optical axis direction, thereby moving the lens holder frame 4 in the optical axis direction.
The position detecting mechanism 7 has an MR magnet 7A mounted on one of the bearings 4A of the lens holder frame 4 and having different magnetic poles alternately arranged in the optical axis direction, and an MR sensor (magnetoresistive device) 7B mounted on the base 2 for detecting magnetic forces of the MR magnet 7A.
When the lens holder frame 4 moves in the optical axis direction, the MR sensor 7B detects magnetic forces of the different magnetic poles of the MR magnet 7A and generates a detected signal. A detecting circuit, not shown, determines the distance that the lens holder frame 4 has moved, i.e., the position thereof in the optical axis direction, based on a change in the detected signal.